feat: add Udemy courses and fix footer i18n + golangci-lint errors

- Add 5 Udemy courses with PDF certificate links (Go, Fyne, HTMX)
- Fix cv-footer.html to use CV data instead of hardcoded values
- Add i18n labels for footer (linkedin, github, domestika, email, phone)
- Fix 11 golangci-lint errors:
  - errcheck: proper Close() error handling in email/security/tests
  - staticcheck: simplify return and merge variable declaration
  - unused: remove legacy sendEmail and formatMessage functions

internal/handlers/cv_text.go

@@ -44,11 +44,7 @@ func isTextBrowser(r *http.Request) bool {
 
 	// Check Accept header - if client prefers text/plain
 	accept := r.Header.Get("Accept")
-	if strings.HasPrefix(accept, "text/plain") {
-		return true
-	}
-
-	return false
+	return strings.HasPrefix(accept, "text/plain")
 }
 
 // ==============================================================================
@@ -72,10 +68,7 @@ func (h *CVHandler) PlainText(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Check icons parameter (default: true)
-	showIcons := true
-	if r.URL.Query().Get("icons") == "false" {
-		showIcons = false
-	}
+	showIcons := r.URL.Query().Get("icons") != "false"
 
 	// Prepare template data using shared helper (loads CV data)
 	data, err := h.prepareTemplateData(langCode)